Our objective is to investigate the regulation of genes encoding proteins important in the function of astrocytes and in neuron-glial interaction. To this end, we have screened human and rat brain expression libraries for cDNAs encoding enzymes of the GABA-glutamate--glutamine cycle. By screening with antibodies, we have isolated cDNA clones for human glutamate dehydrogenase (GDH) and rat glutaminase (GA). Using these cDNAs as probes and cDNAs for glutamine synthetase (GS) and glutamate decarboxylase (GAD), we have compared mRNA levels for these enzymes in various brain region in liver, and in kidney. We have established that GS mRNA is induced in C6 glioma cells by dexamethasone, a glucocorticoid. In a rat model for hepatic encephalopathy, we have shown that brain mRNA levels for GS, GDS, and GA are altered. Using the gene for glial fibrillary acidic protein (GFAP) as a marker, we have shown that immunofluorescence is much less reliable than immunoblots to determine the presence of GFAP in human gliomas. One cell line was developed which shows 20 fold induction of GFAP synthesis after addition of mycophenolic acid. Several nuclear proteins of neurons and astrocytes were identified, two of which significantly increased in amount when astrocytes where cultured under conditions that in vivo cause neurological disorders (0.4 M ethanol or 3.8 mM ammonia). Two abundant plasma membrane proteins present in both neurons and astrocytes but absent in membranes of kidney and liver cells were identified and purified.